Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 6.3.5 - Multiphase CFD Simulations of Chemical Looping Reactors for CO2 Capture
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 Material Information
Title: 6.3.5 - Multiphase CFD Simulations of Chemical Looping Reactors for CO2 Capture Industrial Applications
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: O'Brien, T.J.
Mahalatkar, K.
Kuhlman, J.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: multiphase flow
CFD simulations
carbon capture
chemical looping
reactive flow
 Notes
Abstract: The atmospheric CO2 concentration has increased ~30% since the industrial revolution began (Karl and Trenberth, 2003) due to fossil fuels combustion and, to a lesser extent, land use changes, e.g., deforestration. Carbon capture and sequestration has been proposed as a means to ameliorate anthropogenic climate change but CO2 capture is, itself, an energy intensive process. Chemical looping was first proposed as a means of enhancing efficiency by eliminating the large irreversibility associated with flame combustion (Knoche et al. 1968). Later, it became apparent that CL was a technology that inherently produced a sequestration-ready CO2 product stream without expending significant energy in a gas separation process (Ishida et al. 1994). In this process, an oxygen carrier is oxidized in an air reactor and then separated from the vitiated air stream by a cyclone and introduced into a fuel reactor. In the fuel reactor, the carrier contacts the fuel and oxidizes it, being itself reduced. It is then recycled to the air reactor. In the past decade, there has been an ever increasing amount of work on CL, mostly utilizing gaseous fuels: CH4, natural gas or syngas (Lyngfelt et al. 2001; Abad et al. 2006). However, recently work has been reported on the direct, in-situ utilization of solid fuels in the fuel reactor of a CLC process, which presents new challenges for the development of the technology (Scott et al. 2006; Berguerand et al. 2008). The major challenge of in-situ utilization is that the two feed streams to the fuel reactor, oxidized carrier and fuel, are both granular. Thus, some CO2 and H2O must be recycled to fluidize the unit. However, since the products of reaction are gaseous, mainly CO2 and H2O, there will be extensive self-fluidization as the combustion proceeds. However, direct solid-solid reactions will not occur at a reasonable rate so that, as in many other industrial processes, the reaction between the solid materials is mediated by gas phase species. Specifically, most solid fuels will react in several steps. The first steps, as the fuel heats to bed temperature, are drying and devolatilization. In these steps, H2O, CO, CH4, tar, etc., are rapidly released, as in more conventional combustion technologies. Subsequently, the residual char must be gasified by H2O and CO2. The gaseous products of these reactions must then contact the oxygen carrier before leaving the fuel reactor in order to be reduced. All of these effects have been incorporated into CFD simulations of a typical CL fuel reactor.
General Note: The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows
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Volume ID: VID00153
Source Institution: University of Florida
Holding Location: University of Florida
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Resource Identifier: 635-OBrien-ICMF2010.pdf

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Paper No 7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010



Multiphase CFD Simulations of Chemical Looping Reactors for CO2
Capture


Thomas J. O'Brien1, Kartikeya Mahalatkar2'3 and John Kuhlman2

1National Energy Technology Lab., 3610 Collins Ferry Rd., Morgantown, WV, 26507, USA
2West Virginia University, Dept. of Mechanical and Aerospace Engineering, Morgantown, WV, 26506,
USA
3ANSYS Inc., 3647 Collins Ferry Rd., Suite A, Morgantown, WV, 26505, USA
Thomas.OBrien netl.doe.gov, Kartikeya.Mahalatkar @ansys.com and John.Kuhlman@mail.wvu.edu



Keywords: multiphase flow, CFD simulations, carbon capture, chemical looping, reactive flow




Abstract

The atmospheric CO2 concentration has increased -30% since the industrial revolution began (Karl and
Trenberth, 2003) due to fossil fuels combustion and, to a lesser extent, land use changes, e.g.,
deforestration. Carbon capture and sequestration has been proposed as a means to ameliorate
anthropogenic climate change but CO2 capture is, itself, an energy intensive process. Chemical looping
was first proposed as a means of enhancing efficiency by eliminating the large irreversibility associated
with flame combustion (Knoche et al. 1968). Later, it became apparent that CL was a technology that
inherently produced a sequestration-ready CO2 product stream without expending significant energy in a
gas separation process (Ishida et al. 1994). In this process, an oxygen carrier is oxidized in an air reactor
and then separated from the vitiated air stream by a cyclone and introduced into a fuel reactor. In the fuel
reactor, the carrier contacts the fuel and oxidizes it, being itself reduced. It is then recycled to the air
reactor. In the past decade, there has been an ever increasing amount of work on CL, mostly utilizing
gaseous fuels: CH4, natural gas or syngas (Lyngfelt et al. 2001; Abad et al. 2006). However, recently
work has been reported on the direct, in-situ utilization of solid fuels in the fuel reactor of a CLC process,
which presents new challenges for the development of the technology (Scott et al. 2006; Berguerand et
al. 2008). The major challenge of in-situ utilization is that the two feed streams to the fuel reactor,
oxidized carrier and fuel, are both granular. Thus, some CO2 and H20 must be recycled to fluidize the
unit. However, since the products of reaction are gaseous, mainly CO2 and H20, there will be extensive
self-fluidization as the combustion proceeds. However, direct solid-solid reactions will not occur at a
reasonable rate so that, as in many other industrial processes, the reaction between the solid materials is
mediated by gas phase species. Specifically, most solid fuels will react in several steps. The first steps, as
the fuel heats to bed temperature, are drying and devolatilization. In these steps, H20, CO, CH4, tar, etc.,
are rapidly released, as in more conventional combustion technologies. Subsequently, the residual char
must be gasified by H2O and CO2. The gaseous products of these reactions must then contact the oxygen
carrier before leaving the fuel reactor in order to be reduced. All of these effects have been incorporated
into CFD simulations of a typical CL fuel reactor.






Paper No


Introduction

Due to the threat of global warming, the global
community will be faced with several choices in order to
limit the release of greenhouse gases such as CO2: 1)
conservation significant modification of life style and
the economy to reduce their energy intensity; 2)
efficiency an increase in efficiency of fuel conversion
and utilization; 3) fuel switching an increase in
alternative (non-fossil fuel based) power production
(solar, nuclear, biomass, wind, tidal, geothermal or
hydro) and; 4) carbon capture and sequestration (CCS)
of the CO2 released from continued use of fossil fuels.
Moreover, there are real, physical limits on how fast
new, low-carbon energy technologies can be developed
and deployed to ameliorate the situation. Thus, until
alternative sources of energy are developed on a massive
scale, it seems that the continued use of fossil fuels is
still essential (Hoffert et al. 2002; Kramer et al. 2009).
Therefore, new ways to reduce CO2 emissions from
combustion of fossil fuels have to be developed.
Presently several technologies, like oxy-fuel
combustion, post-combustion capture from flue gases,
carbon shift, etc., are being demonstrated (Simbeck
1998) for CO2 capture. However these technologies will
lead to significant increases in the cost of electricity; a
large portion of the energy they generate is required for
the separation gases (Audus et al. 1998; Bolland et al.
1998; Simbeck 1998). For these technologies, it is the
separation of gases, either CO2 from a flue gas stream
for post-combustion capture or 02 from an air stream for
oxy-combustion, that is estimated to be the major cost
penalty (75% of the energy penalty; 100-200 $/ton C),
rather than compression, transportation and terrestrial
sequestration (25% of the energy penalty; 4-8 $/ton C)
which is usually considered to be the most expensive
component of CCS.
Recently, a new technology called Chemical Looping
Combustion (CLC) has been proposed which would
allow the formation of a concentrated CO2 stream from
the combustion of fossil fuels without the parasitic
expenditure of a significant portion of the produced
energy for separation. This was first proposed as a
combustion technology because of its potential for
higher efficiency (Knoche et al. 1968; Ishida 1983;
Richter et al. 1983). However, it was then realized
(Ishida et al. 1994) that the process inherently produced
a separate CO2/H20 effluent stream, from which the H20
would condense on compression, leaving a
sequestration-ready CO2 stream. Development of the
technology rapidly accelerated through the efforts at
Chalmers University (Lyngfelt et al. 1999; Mattisson et
al. 2000). Since that time there has been a rapid
explosion of research in this area.
However, most of this work has focused on the
utilization of gaseous fuels, including syngas (Jin et al.
2004; Cao et al. 2006). Since the US power industry
currently bums over a billion tons of coal per year, and
the US has large coal reserves, the in-situ utilization of
coal, which promises to be more efficient, is of great
interest (Scott et al. 2006). This is a significantly more
challenging task, but has been demonstrated on the lab
scale (Leion et al. 2007; Yang et al. 2007; Berguerand et


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

al. 2008), and will soon be tested at the 1 MWth pilot
scale (Strohle et al. 2010).

Nomenclature


AR
FR
12D
Pfic
Umf
utr
Us


air reactor
fuel reactor
2nd invariant of the granular stress matrix
frictional granular pressure (Pa)
minimum fluidization velocity (m s 1)
terminal velocity (m s 1)
solids velocity (m s 1)


Greek letters
lffric frictional granular viscosity(Pa s)
Tfric frictional granular stress (Pa)


The Chemical Looping Combustion Process

In its usual configuration, a CLC system consists of two
fluidized bed reactors, an air reactor (AR) and a fuel
reactor (FR) (See Figure 1). The fuel is oxidized in the
FR by a hot granular oxygen carrier, typically a metal
oxide. This process is usually endothermic, but for some
fuel/carrier combination it can be exothermic. For a
gaseous hydrocarbon fuel, contact with the hot carrier
leads to a heterogeneous redox reaction; the products of
this reaction are just CO2 and H20. Since no air is fed to
the FR, this effluent stream is undiluted by N2, the
desired objective. The reduced carrier particles are
returned to the AR where they are re-oxidized by air, a
strongly exothermic process. The AR is typically a
transport reactor. At its exit, the oxidized carrier is
separated by a cyclone and returned to the FR, providing
the oxygen and heat required to "combust" the fuel.
(This is actually the "oxygen separation" step of the
process, but because the oxygen is bound in a solid form
the separation is quite easy.) The net chemical reaction
and energy release of the over-all process is identical to
that of the conventional combustion of the fuel, being
the algebraic sum of the reactions in the two reactors
according to Hess's Law.

Depleted Air Combustion
02 N2 Products CO2, H O


Air Fuel
02 N2 C. H,
Figure 1: Schematic of Chemical Looping Combustion
(CLC) system (Lyngfelt et al., 2001). An oxygen carrier
is exchanged between two reactors. Fuel is combusted
by the oxidized form of the carrier in the fuel reactor.
The reduced form of the carrier is then returned to the






Paper No


air reactor where it is regenerated in a strongly
exothermic reaction. The total energy release is the
same as that of conventional combustion.
As mentioned, most of the work to date utilizes gaseous
fuels, which can be used as the fluidizing medium in the
FR resulting in direct contact with the carrier. The use of
solid fuels, however, leads to an obvious complication
... both of the feed streams to the FR are solid materials.
In order to provide a fluidization gas, some of the
effluent stream must be recycled. This stream must also
mediate heat and mass transfer between the carrier and
fuel since direct solid/solid heat or mass transfer would
be very slow. The fuel, say coal, first devolatilizes as it is
heated and then must be gasified by the recycled
H20/CO2 in the fuel reactor. The devolatilization and
gasification products (e.g., CO and H2) are then oxidized
by contacting the hot metal oxide. The energy spent on
solid circulation (the only energy cost of separation) is
very small (-0.3%) in comparison with the total energy
released (Lyngfelt et al. 2001). The exhaust stream of
the FR consists, mainly, of CO2 and H20, from which a
sequestration-ready CO2 stream can be formed by
compression since the H20 will condense out.

Cold Flow CFB Simulation

As already mention, CLC requires many unit operations
involving gas-solid or granular flow. A CLC system
generally consists of two reactors and granular material
is circulated between them. To date, very few CFD
simulations have been performed of a complete
circulating fluidized bed due to the complexities in
geometry, the flow physics and the large computational
effort required. Most CFD studies have limited
themselves to the riser section of the CFB (O'Brien et al.
1993; Guenther et al. 2002; Mao et al. 2004). This
requires the estimate of boundary conditions, such as the
solids circulation rate, since these are not usually
measured. A few simulations of full circulating fluidized
bed have been performed by (Samuelsberg et al. 1996;
Mathiesen et al. 2000). However in these two
dimensional CFD simulations the complex three
dimensional experimental geometry was greatly
simplified and, again, the focus was on the riser. In most
CFB systems, the flow of solids is limited by narrow
flow sections such as pipes, loop seals, etc. In a CLC
system, predicting the right solid circulation rates for an
oxygen carrier is essential as it will directly affect the
amount of fuel that can be burned and the heat transfer
between reactors. In the present study detailed
mathematical submodels have been incorporated to
describe the flow of solid particles both in the dense and
dilute regimes so that an entire CFB system can be
analyzed. The focus in this study is on accurately
predicting the performance of a complete experimental
CLC system (Kronberger et al. 2004). These
experiments were selected since: 1) the geometry is quite
simple and can more easily be approximated as 2D since
the depth is constant; 2) they are cold-flow yet
predicting the solid circulation rates and flow patterns
are the essential first step in simulations of a complete
CLC system; 3) hot, reactive circulating CLC data with
gaseous fuel is available for a reactor with a very similar


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

geometry (Abad et al. 2006). Most CLC studies are
based on a fluidized or fixed bed which is alternated
between operating as a fuel and an air reactor.

Details of the Cold Flow Experiments

The experiments were conducted in a physical model,
using FCC particles fluidized by a combination of
helium and nitrogen (Kronberger et al. 2004), see Figure
1. The fluidizing velocity in the AR (right compartment)
is higher than the terminal velocity of the particles and
therefore carries the particles upwards. At the top of this
section there is a sudden expansion of the vessel, a
disengagement section. In the experimental
configuration, this expansion is in the depth, i.e.,
perpendicular to the plane of the drawing (Fig. 1, left);
there is a slopping back to the top section of the AR. In
order to capture this effect with a two-dimensional
domain, the expansion has been imposed in the plane of
the drawing (Fig. 1, right); the slopping section is to the
right. Particles fall from the disengagement section back
into the AR or into the narrow downcomer section
connecting the AR with the FR, the left compartment.
This region is densely packed and the slow frictional
flow here controls the solids circulation rate. Particles in
the FR are fluidized at a lower superficial velocity than
on the AR side. Particles can pass back to the AR side
through a slot near the bottom of the separating wall,
completing the circulation. Table 1 provides further
details of the experiment.


Mrina


Pathb


k RUM


11
/ hMin f Miha




Actual experimental Simulated
configuration configuration

Figure 2: Full CLC system studied by Kronberger et al.
(2004).

Simulations of the Kronberger et al Experiments

These experimental results were simulated using the
FluentD code (Ansys-Fluent 2009). The Eulerian-
Eulerian options for the simulation of gas-particle flow
as interpenetration continue were activated. The lower
portion of the computational mesh used in the






Paper No


simulations is shown in Figure 3. The critical flow
regions, the downcomer and the return flow slot at the
bottom, were finely resolved; -2500 quadrilateral cells
were used in the simulation. The spatial discretation
scheme used to solve each phase conservation equation
was QUICK. The other equations were solved using the
2nd order up-wind method. The time integration method
was 1st order implicit, using a time step of 0.000025 s,
with 20 iterations per time step.






























rate was controlled by the fictional granular flow in the

densely packed and this restricted flow controlled the
!!!!!!!interactions between particles becomes important.
fictional term to the granular stress:....................
,Figure 3:Loe.pionofthe eshuse
,.1ronbe a. 20.0..).... periments,
,The,........ stn a..n.t ,models
,luent.c.d...rdensega-p cl..ow.




























to the dense packing limit, enduring fictional
interactions between particles becomes important.
Therefore, such regions the Fluentl code adds a
fictional term to the granular stress:
rate was controlle b hefrcioa ganlr lo nh
downcome between teARad h F.Thsrein a
denelypakedan ths rstictd lowconroledth
solis crcultio. Whn hesoidfrctin ecme cos
to the dense packim.ng iiedrigfitoa
interactions between prilsbcmsiprat
Therefre, suh regiomns h Funt od dd
frictional term tothegrnu.laste:


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

The parameter values used were: Fr = 5.0, m = 2, p = 3,
as,min = 0.5 or 0.53, as,max = 0.63, and b = 0.13.

Table 1: System properties for the Kronberger et al.
(2004) experiments.
Width of FR section (mm) 19
Width of AR section (mm) 27
Width of downcomer (mm) 9
Gas composition (N2/He%) 13/87
Average diameter of particles (jim) 70
Average density of particles (kg/m3) 1500
Minimum fluidization velocity (m/s) 0.0027
(Wu and Yen, 1966)
Terminal velocity of particle (m/s) 0.2
Range of FR gas velocity 18-55 x umf
Range of AR gas velocity 1.2-2 x ut
Mass of metal oxide particle bed (g) 43-58

Results and Discussion of the Cold Flow Simulations

Figure 4 compares time record of the computed solid
circulation rates predicted using the two viscoplastic
models described above. Clearly the solid circulation rates
are considerably different. Table 2 compares the computed
average circulation and leakage rates to the Kronberger
experiments. From these studies it is clear that, for the
present geometry and granular material, the frictional
viscosity model proposed by Langroudi, Turek et al.
(2010) is significantly better; the much lower solid
circulation rates are primarily because of the larger
frictional viscosity predicted by it.
Fig. 5 shows examples of the instantaneous contour plots
for solid volume fraction and the magnitude of the solids
velocity. From the volume fraction plot (Fig 4, left) it is
observed that the downcomer is packed and the frictional
resistance of the packed column is reducing the flow of
solids. The solids velocity becomes small in this region
due to this packing (Fig 5, right).



6oo


Tfric -Pfric + Ifric(VUs + (VUs)T)


Two different viscoplastic models were used in the
present study. The first is the default option of the code
(Schaeffer 1987), where the frictional viscosity is given
by ffric = sin 4 Pfric/fif- The second model was a
new formulation developed by (Langroudi et al. 2010;
Langroudi et al. 2010), in which the frictional viscosity
is given by


fric = 2Pfric((sin 0 + b cos I0 Gl)/2)/ )

Here n is taken to be 0.72 and b = 0.13 (Langroudi et al.
2010). In both cases, the frictional pressure is given by
the Johnson and Jackson model (1987),

Pfric= Fr (as as,min)m(as,max as)


S200

0

S-2
i


Time Secondsl
Figure 4: Comparison of solid circulation rates predicted
by Schaeffer model (1987) and that of Langroudi-
Kheiripour et al. (2010). Slot height for the
experiment/simulation was 1.5mm, sup-FR = 18 Umf and
usup-AR = 1.8 Ut.






7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Table 2: Comparison of Experimental and Simulated
Properties

Simulations
Experiment Schaeffer Langroudi et al.
(1987) (2010)

Solid Mass Flux
(k2/mZ/s) 7.017 313.15 20.12
Fuel to Air Reactor
Leakage (%Flow) 12% 60.24% 15.20%
Air to Fuel Reactor
Leakage (%Flow) 3.50% 0% 1.8%

Fig. 5 shows a comparison between the solid circulation
rates, measured at the inlet of the downcomer where the
oscillations in the solid flow rate are the least. Although the
solid circulation rate at is more than 2 times the
experimental value it is dramatically less than that
predicted using the Schaeffer model. This discrepancy
could be due to 3D effects. However, the parameters used
in the frictional model where not evaluated for the
experimental material but were taken from the literature.
Unless these parameters are determined for the actual
experimental material there seems little point in trying to
determine the circulation rate more accurately.
Fig. 7 shows the gas leakage into AR. The comparison with
experiment is reasonable. Fig. 8 shows the gas leakage into
the FR; due to the low percentage of leakage these results
are off by a factor of about 2. It is clear that the
experimental trends are captured very well with the present
friction viscosity model. Given the extremely complex
frictional dissipative phenomena that occurs in the
downcomer it is not surprising that some of the
quantitative values are differ by 50% when compared with
experiments. Also, Kronberger et al. stated that the solid
circulation rates differ by an order of magnitude when
slightly different solid particles are used (e.g. similar size
glass beads). This clearly highlights the complex nature of
granular frictional flow and difficulties in modeling it
accurately.
















Volume fraction Velocity magnitude (m/s)
Figure 5: Instantaneous contour plots at 6 seconds of
particle properties (uFR =18 Umf and uAR = 1.8 Ut).

Details of the Hot Flow Experiments

The experiments of Abad et al. (2006) used a hot flow
reactor of a similar geometry with a carrier consisting of
manganese oxide supported on zirconium oxide. The
gaseous fuel used was methane; this also served to fluidize
the FR. The AR fluidizing velocity is higher than the


a"o
0.7


OS
0.5

0.3
.2


-- Experiments
--Transitionat 0.5
---Transitionat 0.53


0.1


1.2 1- 1.6 L8 2 22
udu,
Figure 6: Comparison of solid circulation rates with
experiments (uFR =18Umf). Two values of transition volume
fraction as,min have been used (0.5 and 0.53) to check the
sensitivity to this parameter.

is


16

14

12

20



* 6


1.2 1.4 1.6 1.8 2 2.2
u.Ju,
Figure 7: Gas leakage into Air Reactor for two values of
transition volume fraction as,min (0.5 and 0.53) with uFR
18 umf).
4

3.5

3


-- Experiments
-*-Transitionat 0.5
-Transitionat 0.53


0.5

0
12 1.4 1.6 1.8 2 2.2
udu,
Figure 8: Gas leakage into Fuel Reactor for two values of
transition volume fraction as,min (0.5 and 0.53) with uFR
18 umf).

terminal velocity of the particles and therefore carries the
particles upwards. As in the cold flow geometry, at the top
of this section there is a sudden expansion which causes
the particles to disengage. Again, as in the cold flow
simulations, the geometry has been modified such that the
area expansion is in the 2D plane being simulated (Fig. 2).


-- Experiments
--Transition at 0.5
-Transition at 0.53


Paper No




--
--





Paper No


Table 3 provides the details of geometry and fluidization
conditions.

Table 3: System properties for experiments of Abad et al.
(2006).
Width of FR (mm) 25
Width ofAR (mm) 40
Width of downcomer (mm) 12
Fluidizing gas in FR 100% CH4
Fluidizing gas in AR 100% Air
Diameter of particles (microns) 150
Density of particles (kg/m3) 2260
Range of FR flow rates (m3/s) 2.5 7.5 x 10-6
Range of AR flow rates (m3/s) 66.7- 91.7x 10-6
Mass of carrier particle bed (g) 300


Details of the Simulation of the
experiments:


Abad et aL (2006)


Three different grids, with corresponding time-steps, were
used in the present study to establish grid independence:
cell-count/time-step = 8822/2x10-4; 17562/lx10-4;
33745/2.5x10 5. The same spatial and temporal
discretization schemes were used as in the cold flow
simulations. The small time steps are required for
numerical stability. The results presented are primarily
using the medium size mesh unless stated otherwise.
The carrier used in the experiments of Abad et al. (2006)
was manganese oxide supported on zirconium oxide. The
carrier reactions were:
Reduction Reaction:
4Mn304 + CH4 12MnO + CO2 + 2H20

Oxidation Reaction:
12MnO + 202 4Mn304

Because of leakage from the FR to the AR, the direct
combustion of methane was also allowed:
CH4 + 202 CO + H20

Son and Kim (2006) suggest that the uniform reaction
model can best represent the heterogeneous chemical
reactions of metal oxides in a CLC system:
dX/dt = k(- X), where X = (m md)/(mo m d).
Here mo is the oxidized form of the carrier, Mn304, and
md is the reduced form, MnO. The coefficient k WAS
represented in Arrhenius form, k = koeE/RT Adanez et al.
(2004) used thermo-gravimetric analysis to determine the
heterogeneous reaction rates for manganese oxide particles
with the same type and percentage of support material as
that in present case. Based on their studies, the following
constants for the reduction reaction were estimated: ko =
2943.515 s1 and E = 104628.4 J/(K-mol).
For the oxidation reaction, Adanez et al. (2004) showed
that the reaction rate is fast and almost constant for the
temperature range of the present experiments. The
coefficient k for the oxidation reaction is estimated to be
0.1.
The rate of consumption of methane (kg/(m3s)) is given by:


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

kR
S---(l-X)p
CH4 (1- X)pavg s
mCH4 M Y--
2MWo

x Y"nMo4 XMn M34 YCH4 MWCH4
VMnoW1MnO Q CH4 _GA
(9.14)
where YCH4 TGA = 0.675 is the methane mass fraction in
the TGA experiments during the reducing phase of the
experiment and R = (m mred )/mred These rates
have been scaled linearly for other concentration values
considered in these simulations. The concentration of
CH4 in TGA experimeitns was 70% (70% CH4 and 30%
H20).
Similarly the reaction rate for the oxidation reaction
(kg/(m3s)) can be shown to be
kRo
_O = X Pavg8s
S M2MWo

YMo + YMo x Mn,3O Ml30 4 Oz
X M"n04 MnO WnooW, ) 02
oM, Wa,,o ]YOY_-TGAMW

where Y2 TGA = 0.23 for air.
DeSouza-Santos (2004), provide global reaction rates
for the oxidation of methane in systems utilizing coal
and biomass as fuel,
CH4 =kCH4 (RT)T 1 CH4 CO
where kH4 = 3.552 x 1014 M3/(kmol s) and E/R =
15700 K.

Results for Abad et al. (2006) experiments

Fig. 9(left) shows the volume fraction of the gas phase at
one time instant. The formation and rise of bubbles can
be clearly seen in the FR. The downcomer region is
completely packed due to dense frictional flow, as for
the cold flow experiments discussed earlier. As before,
the packing in the downcomer limits the solid circulation
rates.


Ioo i ao
Gas volume frac. CH4 CO2
Figure 9: Gas volume fraction and species mole fraction
contour of various gases (T = 1123 K; FR flow rate = 7.5
x 10-6 m3/s; AR flow rate = 83 x 10-6 m3/s.

Fig. 9 also shows examples of the computed mole
fraction of various gases. Fig. 9(center) shows the rapid
consumption of methane as it enters the fuel reactor.
Also, a higher concentration of methane can be seen in






Paper No


the bubble regions indicating that some amount of fuel
by-passes the bed through the bubbles. Fig. 9(right)
shows the mole fraction of CO2. It is low in the inlet
region of the FR but increases with bed height as the
methane reacts with the carrier. Some leakage of CO2
into the AR can be observed.
Fig. 10 shows the variation of outlet CH4 fraction with
change in the FR gas flow rate. At lower flow rates the
time required for the fuel to flow through the FR bed is
larger and, hence, it has more time to react so that the
outlet concentration of fuel is decreased. Also, the
bubble size and frequency are lower at lower flow rates
reducing the amount of fuel escaping through the
bubbles.


4i


* Experiment

-u-Simulation* Coarse Mesh

-Simulation Medium Mesh

--Simulation Fine Mesh


0.01 I
0 2 4 6 8
FM x 1i (m'/s)
Figure 10: Variation in outlet CH4 fraction
:. ,,,I CH4+Xco+Xco2)), with change in FR flow rates
(FR flow rate = 2.5 7.5 x 10-6 m3/s and AR flow rates =
83 x 10-6 m3/s).

The trends in outlet CH4 concentration are captured
reasonably well for the entire range of flow rates tested.
The differences in the outlet methane concentrations
from the experimental could be for several reasons: 1)
the method of preparation for the metal oxide used by
Abad et al. (2006) was different from that of Adanez et
al. (2004), resulting in differences in porosity and
available particle surface area of the metal oxide; 2) the
TGA experiments used to determine the chemistry were
carried out at a single, excess concentration of methane
and a linear scaling of the reaction rates was assumed
with respect to concentration of methane; 3) errors in
fluid mechanics, such as bubble size or frequency, which
control by-passing.
Fig. 11 shows the variation of outlet CH4 concentration
with change in operating temperature as well as
associated changes in AR air flow rates. Once again the
CFD model predicts reasonable trends. At low operating
temperature the reaction rates are lower resulting in
higher fuel concentrations at the exit. At a higher
temperature of 1223 K the outlet fuel fraction is less than
1%. The outlet fuel fraction does not vary significantly
with change in air flow rates. This is primarily because
the solids circulation rates, as well the solid inventory in
the FR do not change significantly with changes in air
flow rates, as discussed below.
Fig. 12 shows the variation in solid circulation rates with
changes in the fuel flow rates. The solids are observed to
pack in the downcomer (Fig. 9a). The frictional energy


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

dissipation in the downcomer of the CLC system is the
primary controlling mechanism of the solids circulation
rate. Simulations without any frictional stresses showed
a circulation rates -100x greater than observed. Fig. 12
shows that grid refinement results in more accurate
prediction of solid circulation rates. Also, both the
medium and fine mesh predict a slight increase in the
solid circulation rates with increase in fuel flow. This is
similar to the trend observed in the experiments. The
experimental solid circulation rates were calculated
indirectly from transient increase in oxygen
concentration after stopping the combustion test.
The simulations also capture the slight increase in the
solid circulation rates with increase in air flow (Fig. 13).
The differences in simulated and experimental values are
attributed to reasons discussed above.

Temperature(K)
975 1035 1095 1155 1215 1275


S-1 --


* Experiment-Temperature

-c-Simulation Temperature

* Experiment *AR Flow Rate

- -Simulation AR flow rate


0.01
SO 60 70 80 90 100
Fux 10 (m'/s
Figure 11: Variation in outlet CH4 fraction (XCH4/
(XcH4+Xco+Xco2)), with change in operating
temperature of oven (T = 1073 1223 K) and change in
AR flow rates (AR flow rate = 66.7 91.7 x 10-6 m3/s).

14


4 *


2 -


* Experiment

-mSimulation Coars Mesh

* Simulation- Medium Mesh

-Simulation- Fine Mesh


0 *
0 2 4 6 8
F X 106 (m'/s
Figure 12: Variation in solid circulation rate (kg/s x 103)
with change in fuel reactor flow rates (FR flow rates =
2.5 7.5 x 10-6 m3/s; AR flow rate = 83 x 10-6 m3/s.)

Conclusions

Predicting solid circulation rates accurately CLC
systems is important as it determines the amount of
oxygen supplied for burning the fuel as well as the
energy transfer between the exothermic air reactor and






Paper No


the endothermic fuel reactor. Solids circulation is the
primary means of energy transfer between AR and FR.
It is essential that the viscoplastic models used for
describing the frictional pressure and viscosity are able
to predict the correct viscoplastic stresses. In the
Kronberger experiments, it is reported that the frictional
flow in the downcomer causes it to pack with solids and
thus limit the solid circulation rates. Therefore to match
experimental results this phenomenon had to be
accurately predicted.
It is desirable that the CLC reactor consumes all the
supplied fuel for a high combustion efficiency. This can
be achieved by having more solids inventory in the
system but this will also result in higher power
consumption to fluidize and circulate the solid particles.
Therefore there is a need to achieve an optimum level of
solid inventory. This study has shown that a CFD
analysis can be to accurately capture the methane
consumption at a given solid inventory.

Acknowledgements

The authors gratefully acknowledge the financial support
of the U.S. Department of Energy, Carbon Sequestration
and Gasification Programs administered at the National
Energy Technology Laboratory. KM was provided
support through RDS contract DEAC26-04NT41817.

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